1. Field of Invention
The present invention relates to an installation in a stepper machine for microlithographic processing of semiconductors. More particularly, the present invention relates to a method and a controlling system for preventing the scratching of wafer backs by the fetch arm of a stepper machine.
2. Description of Related Art
FIG. 1 shows wafer holders and part of the structural components of the transporting system of a conventional stepper. The suction head of a fetch arm 110 picks up a wafer 130 on the side sliding arm 120, transports it to a cassette holder 100, and then unloads the wafer 130. All through the transportation, the wafer 130 is held by a vacuum in the suction head, until the wafer has dropped inside the cassette holder 100.
For the above transporting system, there is one major drawback. As the fetch arm 110 moves from position A1 to A2 toward the cassette holder 100, there is a possibility that the wafer 130 may hit the side of the cassette holder 100, causing the wafer 130 to be slightly displaced from the fetch arm 110. Therefore, the contacting area between the back of the wafer and the suction head may be scratched.
FIG. 2 is a block diagram of a controlling circuit for controlling the movement of the fetch arm of a conventional stepper. FIG. 3 is a time diagram for the sequence of actions associated with the movement of a fetch arm in a conventional stepper.
As shown in FIG. 3, the operation of a stepper is roughly divided into three operational stages, namely, picking up a wafer from a cassette holder, performing some microlithographic projections, and returning the wafer to the cassette holder. The fetch arm has three action states, namely, a forward state, a retracting state and a stationery state. The suction head of a fetch arm has two states, namely, a vacuum-release state (a non-vacuum state) and a vacuum-hold state (a vacuum state).
Microlithographic projections may also include other subsidiary steps. Since these steps are not a major concern of this invention, detailed description of their operations is omitted. The relevant issues of this invention are the operation of unloading wafers from the cassette holder to the stepper and returning the wafers from the stepper back to the cassette holder. In particular, the main concern is the operation of returning the wafers to the cassette holder because here is when the wafer back may be scratched. Therefore, controlling actions related to the returning of wafers to the cassette holder are described in detail below with reference to FIGS. 1-3.
The fetch arm controlling system of a conventional stepper includes a first sensor 200, a second sensor 210, an input interface 220, a microcontroller 230, an output interface 240 and a vacuum solenoid valve 250.
The first sensor 200 and the second sensor 210 are used for sensing the position of the fetch arm 110. The first sensor 200 senses whether the fetch arm 110 is in position A1, then generates a first sensor signal S.sub.start accordingly. If the fetch arm 110 is in position A1, a logic high output signal will be generated. On the other hand, if the fetch arm 110 is not in position A1, a logic low output signal will be generated. For example, at time t2, the fetch arm 110 starts to move away from position A1, a logic high signal appears in S.sub.start, and at time t6, the fetch arm 110 has returned to position A1, so a logic low signal reappears in S.sub.start.
The second sensor 210 senses whether the fetch arm 110 is in position A2, then generates a second sensor signal S.sub.stop accordingly. If the fetch arm 110 is in position A2, a logic low output signal will be generated. On the other hand, if the fetch arm 110 is not in position A2, a logic high output signal will be generated. For example, at time t3, the fetch arm 110 has moved to position A2, a logic low appears in Sstop; and at time t5, the fetch arm 110 starts moving away from position A2, so a logic high reappears in S.sub.stop.
As shown in FIG. 2, the microcontroller 230 is a device for receiving the first sensor signal S.sub.start, and the second sensor signal S.sub.stop, and then outputting a first control signal S.sub.c. The first control signal S.sub.c controls the opening or closing of the vacuum solenoid valve 250. The vacuum solenoid valve 250 in turn controls the action of releasing the vacuum or holding the vacuum in the suction head of the fetch arm 110.
FIG. 3 illustrates the steps involved in returning a wafer 130 to a cassette holder 100. First, at time t1, a vacuum is created in the suction head of the fetch arm 110 before the fetch arm starts moving away from position A1 at time t2. At time t3, the fetch arm arrives at position A2 and stops. At time t4, the vacuum in the suction head is released. Hence, during the period from time t1 to t4, the suction head of the fetch arm 110 is in a vacuum state. In other words, wafer 130 is held firmly by the suction head of the fetch arm all through its forward journey. A major drawback for this controlling system is that, should the wafer 130 hit the side of the cassette holder 100 during the forward journey of the fetch arm from position A1 to A2, the wafer 130 may be forced to displace slightly. If the wafer is free to move, there will be no problem. However, since the wafer is held firmly by the suction head through a vacuum, the contacting area between the back of wafer and the suction head may be scratched.
In light of the foregoing, there is a need in the art for preventing the scratching of wafer backs by the fetch arm of a stepper.